Say it is extremely rural and we only have 20 customers in those 30 miles.

So, $4.5 million per customer.

At this point, why be attached to the grid at all?

A renewable setup with storage makes sense at that point.

It makes sense at the current rates. Solar can replace the entire grid. And instead of using large HVAC transmission lines, small communities can form small grids to help share battery stored energy (while still metered so the power usage is fair) or a resident could opt out of the community grid and do fully off grid solar as long as the had the ability to handle a few cloudy days.

Unless you live in an area where electricity prices are sky high and A/C is all but a necessity that's like 50% of people's bills.

That might fly in some rich suburb of NYC but try that in Buffalo and the tar and feathers would come out.

My entire monthly bill from PG&E is usually about $50...

Ya, without the need to run AC or heat (like in SF or even LA outside of summer), electric bills can be pretty cheap.

PG&E has 81,000 miles of distribution lines. Even if they served all of California (they do not), that would be less than 15K customers for an average 30 mile segment.